Magnetic tape and method for producing the same

ABSTRACT

A magnetic tape produced by cutting a magnetic sheet with a wide breadth into a tape having a predetermined width using a cutting device, in which the magnetic sheet has a magnetic layer containing magnetic powder and a hinder formed on one surface of a non-magnetic substrate having a thickness of 4 μm or less, and a center line average height (Ra) along a roughness curve of a cut edge of the magnetic tape is from 0.08 to 0.25 μm.

The present application is a Divisional of copending U.S. applicationSer. No. 13/457,037, filed Apr. 26, 2012, which is a Divisional ofApplication No. 12/535,367, filed Aug. 4, 2009, which claims the benefitof priority from Japanese Patent Application No. 2008-201392, filed onAug. 5, 2008. The entire contents of all of the above applications arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a coating type magnetic tape and amethod for producing the same. Particularly, the present inventionrelates to a magnetic tape suitable for use with a linear serpentinetype magnetic recording-reproducing system.

BACKGROUND ART

In general, a magnetic tape which is a coating type magnetic recordingmedium is produced as follows: a magnetic sheet with a wide breadth isprepared by coating one surface of a non-magnetic substrate with amagnetic coating composition comprising a dispersion of magnetic powder,a binder and optionally other additive(s) in a solvent; drying thiscoating composition to form a magnetic layer thereon; coating the othersurface of the non-magnetic substrate with a coating composition for abackcoat layer comprising a dispersion of a pigment such as carbonblack, a binder and optionally other additive(s) in a solvent; anddrying this coating composition to form a backcoat layer; and thismagnetic sheet is slit into tapes with predetermined widths in a cuttingstep.

Recording-reproducing systems with the use of magnetic tapes are roughlyclassified to a helical scan type using a rotatable magnetic head and alinear serpentine type using a stationary magnetic head. Therecording-reproducing systems of these types have their own advantages,respectively. However, the recording-reproducing systems of the lattertype, i.e., the linear serpentine type, have come into wide practicaluse as the recording-reproducing systems with the use of backup tapes,because a touching condition between the magnetic head and a magnetictape is gentle, and because restriction in the volume of a magnetic tapecartridge is mild. As such commercial products, for example, DigitalLinear Tape (or DLT) and Linear Tape-Open (or LTO) have been put on themarket.

Mass storage magnetic tapes for backup are indispensable to correspondto hard discs of which the storage is increasing year after year. Toincrease the storage capacity of a magnetic tape, it is necessary toincrease its recording area, in other words, to increase the length ofthe magnetic tape per reel by decreasing the total thickness of themagnetic tape. In this regard, the component which occupies the largestproportion of the volume of the entire magnetic tape is a non-magneticsubstrate, and therefore, it is effective to decrease the thickness ofthe non-magnetic substrate so as to increase the length of the magnetictape per reel. In the above-described DLT and LTO, a polyethyleneterephthalate (PET) film or a polyethylene naphthalate (PEN) film havinga thickness of from 5 to 6 μm and higher strength in its lengthwisedirection is used as a non-magnetic substrate. However, from theviewpoint of mass storage, it is necessary to use a non-magneticsubstrate with a far thinner thickness of 4 μm or less: for example, abackup tape comprising a non-magnetic substrate with a thickness of from2 to 3 μm is proposed (cf. JP-A-10-134337).

In the linear serpentine type magnetic recording-reproducing system,generally, a plurality of tracks are formed on the magnetic layer alongthe lengthwise direction. When recording or reproducing of signals isperformed on the magnetic tape, the magnetic head is moved across onetrack from one edge to the other edge of the track in the widthwisedirection. To prevent off-track of the magnetic head, it is necessary toensure the linear running performance of the magnetic tape fed at a highspeed. Therefore, a plurality of guide rollers having flange portionsare provided between the magnetic tape cartridge and the take-up reel inthe magnetic recording-reproducing system. However, this leads to thefollowing disadvantage: when the cut edges of the magnetic tapefrictionally touch the flange portions of the guide rollers during thefeeding of the magnetic tape, decrease in the strength of the magnetictape is unavoidable because of the thinner non-magnetic substrate, sincethe flexural rigidity of the magnetic tape tends to decrease inproportion to the cube of its thickness. As a result, when the cut edgesof the magnetic tape frictionally touch the flange portions of the guiderollers during the feeding of the magnetic tape, the magnetic tape isapt to deform, which makes it hard to ensure the linear runningperformance of the magnetic tape which is passing on the guide rollers.There is another problem: when the thickness of the non-magneticsubstrate decreases, the cutting property of the magnetic sheet isreduced to degrade the smoothness of the cut edges of the magnetic tape.When the magnetic tape obtained from such a magnetic sheet is fed in themagnetic recording-reproducing system described above, a part of one ofthe cut edges of the magnetic tape is likely to strike the flangeportions of the guide rollers, with the result that the magnetic layeror the backcoat layer is damaged to cause flaking therefrom.

In a magnetic tape for use with a low-speed helical scan type magneticrecording-reproducing system, JP-B-07-114012 proposes that flaking froma magnetic layer or a backcoat layer of a magnetic tape is suppressed bycutting a magnetic sheet with a laser beam, so that the cut edges of themagnetic layer or the backcoat layer can be positioned backwardinwardly, from the cut edges of the non-magnetic substrate, in order toprevent such flaking. However, the following problems still remain: evenwhen a magnetic tape comprising a thin non-magnetic substrate and havingsuch cut edges as described above is fed at a high speed in the linearserpentine type magnetic recording-reproducing system, flaking from themagnetic tape at the guide rollers cannot be sufficiently eliminated,and the linear running performance of the magnetic tape cannot beimproved.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic tape whichcomprises a far thinner non-magnetic substrate than any of theconventional ones and which shows superior linear running performancewhile being fed on guide rollers having flange portions at a high speed,with less flaking therefrom, when recording or reproducing of data isperformed on this magnetic tape in a linear serpentine type magneticrecording-reproducing system.

Accordingly, the present invention provides a magnetic tape produced bycutting a magnetic sheet with a wide breadth into a tape with apredetermined width using a cutting device, wherein the magnetic sheetcomprises a magnetic layer containing magnetic powder and a binderformed on one surface of a non-magnetic substrate having a thickness of4 μm or less, and wherein a center line average height (Ra) along aroughness curve of a cut edge of the magnetic tape is from 0.08 to 0.25μm.

According to the magnetic tape of the present invention, the center lineaverage height (Ra) on the cut edge of the magnetic tape is from 0.08 to0.25 μm, and therefore, the magnetic tape has the advantages, that is,even when recording or reproducing of signals is performed on thismagnetic tape in a linear serpentine type magnetic recording-reproducingsystem, the friction coefficient of the cut edge of the magnetic tapeagainst the flange portions of guide rollers during the high-speedfeeding thereof is small, and the striking of one of the cut edges ofthe magnetic tape against the flange portions is suppressed, so thatflaking from the magnetic tape can be reduced, while maintaining thelinear running performance of the magnetic tape.

The magnetic tape of the present invention may further comprise a primerlayer containing inorganic powder and a binder between the non-magneticsubstrate and the magnetic layer. The primer layer, if present, impartsstiffness to the magnetic sheet even though the non-magnetic substrateused therein is thin, so that the cutting property of the magnetic sheetis improved.

Preferably, the binder of the primer layer contains an electronradiation curing resin. The primer layer containing an electronradiation curing resin can further improve the cutting property of themagnetic sheet

A magnetic tape obtained from such a magnetic sheet as described abovecan be advantageously used with a linear serpentine type magneticrecording-reproducing system, because the magnetic tape is excellent inlinear running performance and suffers from less flaking therefrom, evenwhen the magnetic tape is fed at a high speed while frictionallytouching the guide rollers having flange portions.

The present invention also provides a process for producing the magnetictape of the present invention, which process is characterized in that aratio (Ew/Et) of a Young's modulus of a magnetic sheet in the widthwisedirection (Ew) to a Young's modulus thereof in a lengthwise direction(Et) is from 0.85 to 1.15; a cutting device comprises a cutting meanswhich has thin blades and thick blades located confronting the thinblades while intermeshing therewith, a feeding means which carries themagnetic sheet to a position between the thin blades and the thickblades, and a take-up means; a intermeshing depth (L) of the thin bladesand the thick blades is from 0.05 to 0.1 mm, and the maximum surfaceroughness (P-V) of each of the side faces of the thin blades and thethick blades intermeshing with each other is 0.05 μm or less; and themagnetic sheet is cut into tapes with predetermined widths, while beingso conveyed that fluctuation of the magnetic sheet in the widthwisedirection in the proximity of the cutting means is 100 μm or less.

When the magnetic sheet described above and the cutting conditionsdescribed above are employed, the fluctuation of the magnetic sheetwhile being cut becomes smaller, and a shear force from the cuttingmeans can be efficiently applied to the magnetic sheet.

As described above, the present invention can achieve the followingeffects: even when a magnetic tape which comprises a thin non-magneticsubstrate to achieve mass storage is used with a linear serpentine typemagnetic recording-reproducing system, the linear running performance ofthe magnetic tape on the guide rollers can be improved, and flaking fromthe magnetic tape due to frictional contact with the guide rollers canbe reduced.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described in detail with reference to theaccompanying drawings, wherein

FIG. 1 shows a schematic diagram illustrating an example of atape-feeding system in a linear serpentine type magneticrecording-reproducing system in which a magnetic tape according to thepresent invention is used;

FIG. 2 shows an enlarged view of a part of the tape-feeding system shownin FIG. 1, illustrating the frictional touching state of the magnetictape to the guide roller;

FIG. 3 shows a schematic diagram of a cutting device for use in theproduction of the magnetic tape according to the present invention; and

FIG. 4 shows a schematic diagram of the cutting means shown in FIG. 3,and an enlarged view of the essential part thereof.

DESCRIPTION OF REFERENCE NUMERALS

3=a magnetic tape

3 a=a magnetic layer

3 b=a non-magnetic substrate

3 c=a backcoat layer 6=a guide roller

6 a=flange portions

10=a cutting means

11=thin blades

11 a=side faces of thin blades

12=thick blades

12 a=side faces of the thick blades

20=a feeding section

30=a take-up section

L=intermeshing depth

M=a magnetic sheet

S=a cut edge

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the schematic diagram of an example of a tape-feedingsystem in a linear serpentine type magnetic recording-reproducing systemwith which a magnetic tape according to an embodiment of the presentinvention is used. As shown in FIG. 1, the magnetic tape cartridge 1 isa single reel type cartridge in which the magnetic tape 3 is wound ontothe single tape reel 2. In this magnetic recording-reproducing system,the door 4 on the front side of the magnetic tape cartridge 1 is openedupon the insertion of the magnetic tape cartridge 1 into the cassettecompartment 5 of the magnetic recording-reproducing system, and themagnetic tape 3 is pulled out by a tape-pulling means in the system,through the opened door 4. Then, the pulled magnetic tape 3 is led by aplurality of the guide rollers 6 each having flange portions on theupper and lower sides, and is then wound onto the take-up reel 7.

FIG. 2 shows the schematic sectional view of the guide roller 6,illustrating the frictional touching state of the magnetic tape 3 to theguide roller 6 in the tape-feeding system shown in FIG. 1. In thismagnetic recording-reproducing system, the backcoat layer 3 c formed onthe surface of the non-magnetic substrate 3 b which has the magneticlayer 3 a formed on the opposite surface thereof comes into contact withthe surface of the hub of the guide roller 6. When the magnetic tape 3is fed at a high speed in this magnetic recording-reproducing system,the magnetic tape 3 tends to move up and down so that the cut edges S ofthe magnetic tape 3 are likely to frictionally touch the upper and lowerflange portions 6 a of the guide roller. Therefore, flaking from themagnetic tape 3 containing the thin non-magnetic substrate 3 b is apt tooccur, or the magnetic tape tends to deform. This may lower the linearrunning performance of the magnetic tape. Such disadvantages are seriousin the linear serpentine type magnetic recording-reproducing system inwhich a lot of guide rollers 6 are provided to ensure the linear runningperformance of the magnetic tape 3.

One of factors to cause flaking from the magnetic tape containing thethin non-magnetic substrate is considered to be poor smoothness of thecut edges of the magnetic tape, when the magnetic tape is cut from amagnetic sheet. This is described in detail. When the magnetic sheet iscut, the rigidity of the magnetic sheet tends to lower with a decreasein the thickness of the non-magnetic substrate. Therefore, the magneticsheet is apt to move when cut, and therefore, the cut edges of themagnetic tape become rough. When the raised portions of one of the roughcut edges of the magnetic tape strike the flange portion of the guideroller, a stress tends to concentrate on the raised portions of the cutedge, resulting in the occurrence of flaking from such portions of themagnetic tape. On the other hand, such concentration of a stress can beavoided if the cut edges are smoothened, while the contact area betweenthe cut edge and the flange portion is increased, which leads to higherfriction. As a result, it becomes difficult to ensure the linear runningperformance of the magnetic tape during the high-speed feeding thereof,if the non-magnetic substrate used is so thin as to be liable to deformdue to the frictional contact with the flange portion.

Under such a situation, the present inventors have made studies on suchcut edges of a magnetic tape that make it possible to reduce the flakingfrom the magnetic tape due to the frictional contact with the flangeportions, of guide roller and also make it possible to ensure the linearrunning performance of the magnetic tape on the flange portions of theguide roller. As a result, it has been found that the problems can besolved when the center line average height (Ra) along the roughnesscurve of the cut edge of the magnetic tape is adjusted to 0.08 to 0.25μm: that is, under this condition, even if a magnetic tape comprising anon-magnetic substrate having a thickness of 4 μm or less runs at a highspeed, the frictional contact between the cut edge of the magnetic tapeand the flange portion of the guide roller becomes less, the flakingfrom the magnetic tape can be reduced, and the linear runningperformance of the magnetic tape can be ensured. When the center lineaverage height (Ra) is smaller than 0.08 μm, the smoothness of the cutedge of the magnetic tape becomes better, while the friction coefficientof the cut edge to the flange portion of the guide roller tends toincrease, and consequently, it becomes difficult to ensure the linearrunning performance of the magnetic tape, when the non-magneticsubstrate thereof is thin and thus is liable to deform. On the otherhand, when the center line average height (Ra) is larger than 0.25 μm,the friction coefficient of the cut edge of the magnetic tape to theflange portion of the guide roller decreases, while there occurs localstress concentration on the raised portions of the cut edge of themagnetic tape, with the result that flaking from the magnetic tape ismore likely to occur. Hitherto, it has not been proposed that, byrestricting the roughness of the cut edges of a magnetic tape asspecified above, a magnetic tape can be provided, which has a thinnon-magnetic substrate and which can achieve linear running performanceand also can reduce flaking therefrom when it is fed in a linearserpentine type magnetic recording-reproducing system. In this regard,the center line average height (Ra) along the roughness curve of the cutedge of the magnetic tape is a value which is determined based on themeasurement with a laser microscope.

Next, a preferable process for producing a magnetic tape having theabove-described cut edges will be concretely described.

FIG. 3 shows the schematic diagram of an example of a cutting device foruse in the production of a magnetic tape according to the embodiment ofthe present invention. FIG. 4 shows the front view of the cutting meansshown in FIG. 3, illustrating the structure thereof, and also shows theenlarged view of the essential portion thereof

As shown in FIG. 3, the cutting device comprises a feeding section 20which includes a cutting means 10 for cutting a magnetic sheet M intotapes with predetermined widths, a feeding roll 21 onto which themagnetic sheet M is wound, and a plurality of conveyer rollers 22 forconveying the magnetic sheet M from the feeding roll 21 to the cuttingmeans 10; and a take-up section 30 which includes a take-up roll 31 fortaking up a magnetic tape 3, and a plurality of conveyer rollers 32 forconveying the magnetic tape 3 cut from the magnetic sheet, from thecutting means 10 to the take-up roll 31. The number of the take-upsections 30 may be increased corresponding to the number of the magnetictapes 3 cut from the magnetic sheet M.

As shown in FIG. 4, the cutting means 10 comprises upper rotatable thinblades 11 and lower rotatable thick blades 12 which are locatedconfronting each other so as to intermesh with each other at apredetermined intermeshing depth (L). When the magnetic sheet M is cut,the thin blades 11 and the thick blades 12 are synchronously rotated bya rolling mechanism comprising a motor (not shown), etc., while the sidefaces 11 a of the thin blades 11 being pushed against the side faces 12a of the thick blades 12. The velocities of the tip ends of the thinblades 11 and the thick blades 12 are so controlled as to besubstantially equivalent to the feeding speed of the magnetic sheet Mconveyed from the feeding section 20 to the cutting means 10.

It is necessary to select the cutting conditions for cutting themagnetic sheet M by the use of the above-described cutting device sothat the shear forces from both the blades 11 and 12 can be efficientlyapplied to the magnetic sheet M which is kept unmovable as much aspossible. In other words, it is necessary to control a web tension withthe feeding roll 21, the take-up roll 31 and the conveyer rollers 22 and32 so as to efficiently apply the shear forces from both the blades 11and 12 to the magnetic sheet M, when the magnetic sheet M is cut.However, the following problem arises when a thin non-magnetic substrateis used in the magnetic sheet: that is, because of .its poor mechanicalstrength, the non-magnetic substrate has low stiffness and it is liableto stretch in the lengthwise direction rather than the widthwisedirection, and therefore, it becomes difficult for the magnetic tape tohave cut edges with roughness within the above-specified range.According to the present inventors' studies, it has been revealed that amagnetic tape having cut edges with center line average heights (Ra)within the above-specified range can be obtained by satisfying thefollowing four conditions.

The first condition is to use a magnetic sheet M having isotropicelasticity, in which the ratio Ew/Et of the Young's modulus in thewidthwise direction (Ew) to the Young's modulus in the lengthwisedirection (Et) is from 0.85 to 1.15. When the ratio Ew/Et is outside theabove-specified range, the elasticity of the magnetic sheet becomesanisotropic, and thus, the magnetic sheet largely stretches in onedirection and thus is apt to fluctuate while being conveyed.

The second condition is to adjust the intermeshing depth (L) of the thinblades 11 and the thick blades 12 within a range of from 0.05 to 0.1 mm.When the intermeshing depth (L) is outside the above-specified range,the magnetic tape 3 tends to stretch in one direction, and thesmoothness of the cut edges of the magnetic tape tends to degrade.

The third condition is to use the thin blades 11 and the thick blades 12which satisfy the following condition: the maximum surface roughness(P-V) of each of the side faces 11 a and the side faces 12 a at whichthe thin blades 11 and the thick blades 12 intermesh with each other is0.05 μm or less. When the maximum roughness (P-V) of each of the sidefaces 11 a and the side faces 12 a is outside this range, the cut edgesof the magnetic tape tend to crack, and the smoothness thereof tends todegrade. In this regard, the maximum surface roughness (P-V) indicatesthe distance between the maximum peak and the maximum valley within themeasured range of the roughness curve found when the side face of eachblade is optically evaluated.

The fourth condition is to constantly maintain the fluctuation of themagnetic sheet M in the widthwise direction at 100 μm or less in theproximity of the cutting means 10, by controlling the web tension of themagnetic sheet M. When a high web tension is applied to the magneticsheet during the cutting step, the magnetic sheet is liable to snake andis cut while being drawn in the lengthwise direction. Consequently, thecutting condition differs between the largely drawn portion of themagnetic sheet and the poorly drawn portion thereof. When a continuousmagnetic tape is produced, it becomes difficult to restrict the centerline average height (Ra) within the center line above-specified rangeover the entire length of the magnetic tape, accordingly. In thisregard, the fluctuation of the magnetic sheet can be detected with adetecting means 40 such as an edge position control (EPC) which isdisposed in the proximity of the cutting means 10.

Next, the components such as a non-magnetic substrate and a magneticlayer suitable for use in the magnetic tape of the present inventionwill be described in detail.

As the non-magnetic substrate, a thin polymeric resin film with athickness of 4 μm or less is used for increasing the mass storagecapacity. The thinner the non-magnetic substrate, the more preferable itis. However, it is difficult for the non-magnetic substrate to have auniform thickness, if the thickness of the non-magnetic substrate is toothin. Thus, the thickness of the non-magnetic substrate is preferably atleast 1 μm. Specific examples of such a non-magnetic substrate include apolyamide film, a polyamide film, etc. To produce the magnetic sheetwith the above-described Young's moduli isotropic in the lengthwise andwidthwise directions, preferably, the non-magnetic substrate has aYoung's modulus of from 8 to 20 GPa in the widthwise direction and aYoung's modulus of from 9 to 22 GPa in the lengthwise direction.

The magnetic layer comprises magnetic powder and a binder. Specificexamples of the magnetic powder include ferromagnetic iron oxidemagnetic powder, cobalt-containing ferromagnetic iron oxide magneticpowder, hexagonal ferrite magnetic powder, ferromagnetic metallic ironmagnetic powder, iron nitride magnetic powder, etc. Among those,hexagonal system ferrite magnetic powder and iron nitride magneticpowder are preferable, and iron nitride magnetic powder are mostpreferable, since a magnetic layer with Young's moduli small inanisotropy can be formed therefrom. Such iron nitride magnetic powder isdescribed in detail in, for example, JP-A-2004-273094.

The binder of the magnetic layer may be a conventional thermoplastic orthermosetting resin. Specific examples of the thermoplastic resininclude polymers or copolymers, any of which contains, as a constitutiveunit, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylicacid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylicacid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral,vinyl acetal, vinyl ether or the like. Specific examples of thethermosetting resin include phenol resins, epoxy resins, polyurethaneresins, urea resins, melamine resins, alkyd resins, etc. The content ofany of these binders for use in the magnetic layer is preferably from 5to 50 parts by weight per 100 parts by weight of the magnetic powder.

The binder may be used in combination with a thermocuring crosslinkingagent which is bonded to functional groups, etc. in the binder to form acrosslinked structure. Specific examples of the crosslinking agent areisocyanate compounds such as tolylenediisocyanate,hexamethylenediisocyanate and isophoronediisocyanate; reaction productsof isocyanate compounds with compounds each having a plurality ofhydroxyl groups, such as trimethylolpropane, etc.; and polyisocyanatessuch as condensation products of isocyanate compounds. The crosslinkingagent is used usually in an amount of from 10 to 50 parts by weight per100 parts by weight of the binder.

The magnetic layer may contain an additive or additives such as carbonblack, a lubricant and non-magnetic powder in order to improve theelectric conductivity, surface smoothness and durability thereof.Specifically, acetylene black, furnace black or thermal black is used ascarbon black. The content of carbon black is preferably from 0.2 to 5parts by weight per 100 parts by weight of the magnetic powder. Specificexamples of the lubricant include fatty acids, fatty acid esters andfatty acid amides, each having 10 to 30 carbon atoms. The content of thelubricant is preferably from 0.2 to 3 parts by weight per 100 parts byweight of the magnetic powder. Specific examples of the non-magneticpowder include alumina, silica, etc. The content of the non-magneticpowder is preferably from 1 to 20 parts by weight per 100 parts byweight of the magnetic powder.

The thickness of the magnetic layer is preferably 300 nm or less, morepreferably from 10 to 300 nm, still more preferably from 10 to 250 nm,most preferably from 10 to 200 nm, from the viewpoint of higherrecording density. When the thickness of the magnetic layer exceeds 300nm, reproduction output tends to decrease due to thickness loss. Whenthe thickness of the magnetic layer is smaller than 10 nm, the magneticlayer is hard to have uniformity in thickness. Preferably, the Young'smodulus of the magnetic layer in the widthwise direction is from 4 to 10GPa, and the Young's modulus thereof in the lengthwise direction is from4 to 9 GPa.

When the magnetic sheet is produced, the above-described magnetic powderand binder and optionally other additives are mixed in a solvent toprepare a magnetic coating composition, this magnetic coatingcomposition is applied to the non-magnetic substrate, and the appliedmagnetic coating layer is oriented and dried to provide the magneticsheet. As the solvent, any of the organic solvents used for thepreparation of the conventional magnetic coating compositions may beused. Specific examples of the solvent include cyclohexanone, toluene,methyl ethyl ketone, tetrahydrofuran, etc. For the preparation of themagnetic coating composition, any of the coating composition-preparingmethods employed for the production of the conventional magneticrecording media may be employed. Particularly preferable is a methodcomprising a kneading step with the use of a kneader, in combinationwith a primary dispersing step. In the primary dispersing step, a sandmill is preferably used, since the dispersibility of the coatingcomposition is improved, and since the surface properties of themagnetic layer can be controlled.

The magnetic tape of the present invention may have at least one primerlayer containing inorganic powder and a binder between the non-magneticsubstrate and the magnetic layer. The thickness of the primer layer ispreferably from 0.1 to 3.0 mm, more preferably from 0.15 to 2.5 μm.Particularly preferable is the primer layer containing inorganic powderwhich comprises acicular inorganic particles having an average majoraxial length of from 30 to 100 nm, and an average axis ratio of from 2to 4. The formation of the primer layer containing such acicularinorganic particles is effective to impart stiffness to the magneticsheet and to improve the cutting property of the magnetic sheet.Examples of such inorganic powder include non-magnetic powders such asiron oxide and alumina, and magnetic powders such as y-iron oxide,Co-y-iron oxide, magnetite, chromium oxide, Fe—Ni alloys, Fe—Co alloysand Fe—Ni—Co alloys. Each of these inorganic powders may be used alone,or two or more of them may be used as a mixture.

As the binder of the primer layer, there may be used any of the samethermoplastic and thermosetting resins as those exemplified as thebinders of the magnetic layer. More preferably, there is used anelectron radiation curing resin which releases a radical when exposed toan electron ray and which has at least one unsaturated double bond inthe molecule curable by crosslinking or polymerization. Generally, theprimer layer is thicker than the magnetic layer in a high recordingdensity magnetic tape, and therefore, the rigidity of the primer layerdominantly affects the rigidity of the magnetic sheet. If the primerlayer contains the electron radiation curing resin superior in rigidityto the thermoplastic and thermosetting resin as the binder, iteffectively improves the stiffness of the magnetic sheet to improve thecutting property of the magnetic sheet. The content of the bindercontained in the primer layer is preferably from 7 to 50 parts byweight, more preferably from 10 to 35 parts by weight, per 100 parts byweight of the inorganic powder.

Examples of the electron radiation curing resin are the followingresins, each of which is modified to have electron radiation sensitivityby introducing a (meth)acrylic double bond therein in a known manner:that is, such resins are vinyl chloride resins and polyurethane resins,(meth)acrylic resins, polyester resins, acrylonitrile-butadienecopolymers, polyamide resins, polyvinylbutyral, nitrocellulose,styrene-butadiene copolymers, polyvinyl alcohol resins, acetal resins,epoxy resins, phenoxy resins, polyether resins, polyfunctionalpolyethers such as polycaprolactone, polyamide resins, phenol resins,polybutadiene elastomers, chlorinated rubber, acrylic rubber, isoprenerubber and epoxy-modified rubber. Preferable among those is an electronradiation curing resin which is obtained by using a vinyl chloride resinor a polyurethane resin as a starting material, and modifying such astarting material with a compound having an isocyanate group and aradical polymerizable unsaturated double bond in the molecule, under anatmosphere having a specified moisture content, so as to impart electronradiation sensitivity to the starting material.

To impart electric conductivity and surface lubricity to the magneticlayer, the primer layer preferably contains carbon black and alubricant. As the carbon black and the lubricant, the same materials asthose used for the magnetic layer may be used. The method for preparinga coating composition for the primer layer may be the same method asemployed for the magnetic coating composition. When the primer layer isformed, the magnetic coating composition and the primer layer coatingcomposition may be applied by any of the method of sequentialapplication for lamination of layers and the method of concurrentapplication for lamination of layers (a wet-on-wet method). When theprimer layer is formed, preferably, the

Young's modulus of the lamination of the magnetic layer and the primerlayer in the widthwise direction is from 4 to 10 GPa, and the Young'smodulus thereof in the lengthwise direction is from 4 to 9 GPa.

The magnetic tape of the present invention may have a backcoat layer onthe other surface of the non-magnetic substrate having the magneticlayer formed on one surface. The thickness of the backcoat layer ispreferably from 0.2 to 0.8 μm, more preferably from 0.3 to 0.8 μm.Preferably, the backcoat layer contains carbon black such as acetyleneblack, furnace black or thermal black. As the binder in the backcoatlayer, any of the resins used in the magnetic layer may be used.Preferably, a cellulose resin is used in combination with a polyurethaneresin, so as to decrease the friction coefficient of the magnetic tapeand to improve the running performance of the magnetic tape. When thebackcoat layer is formed, preferably, the Young's modulus of thebackcoat layer in the widthwise direction is from 4 to 10 GPa, and theYoung's modulus thereof in the lengthwise direction is from 4 to 9 GPa.

The Young's moduli of the entire magnetic sheet in the widthwise andlengthwise directions are not particularly limited, insofar as the ratioof the Young's, moduli in the widthwise and lengthwise directions fallswithin the ranges specified above. This is because the Young's moduli ofthe magnetic sheet vary depending on the kind and thickness of thenon-magnetic substrate, the composition and thickness of the magneticlayer and the presence or absence of the primer layer and/or thebackcoat layer. Preferably, the Young's modulus of the entire magneticsheet in the widthwise direction is from 4 to 10 GPa, and the Young'smodulus thereof in the lengthwise direction, from 4 to 10 GPa.

The magnetic tape of the present invention is produced by cutting themagnetic sheet obtained as above under the cutting conditions describedabove. Even if the magnetic tape thus obtained contains a thinnon-magnetic substrate, it has excellent linear running performance whenit is fed at a high speed, and flaking therefrom is reduced. Therefore,it is possible to lengthen this magnetic tape as a continuous one andthus is possible to obtain a mass storage magnetic tape. The thinner theentire thickness of the magnetic tape, the more preferable it is,because a larger storage capacity can be obtained. The entire thicknessof the magnetic tape is preferably from 2 to 8 μm, more preferably from3 to 6 μm.

EXAMPLES

Hereinafter, the present invention will be described in more detail byway of Examples thereof, which should not be construed as limiting thescope of the present invention in any way. In the Examples, “part” means“part by weight”, unless otherwise specified.

Preparation of Magnetic Coating Composition (I)

The components of a magnetic coating composition (I) shown in Table 1below were kneaded with a kneader, the kneaded mixture was dispersedwith a sand mill (residence time: 60 minutes), and the resulting liquiddispersion was mixed with polyisocyanate (6 parts), and the mixture wasfurther stirred and filtered to obtain the magnetic coating composition(I).

TABLE 1 Components of Magnetic Coating Composition (I) Amount (parts)Fel6N2 phase-containing iron nitride magnetic powder 70 (Y and Al added)(average particle size: 15.5 nm) Vinyl chloride-hydroxypropylmethacrylate copolymer 15 resin (—SO₃Na groups contained: 0.7 × 10⁻⁴eq./g) Polyester polyurethane resin 9 (—SO₃Na groups contained: 1.0 ×10⁻⁴ eq./g) α-Alumina (average particle size: 80 nm) 4 Cyclohexanone 156Toluene 156

Preparation of Primer Layer Coating Composition (i)

The components of a primer layer coating composition (i) shown in Table2 below were kneaded with a kneader; the kneaded mixture was dispersedwith a sand mill (residence time: 60 minutes) and the resulting liquiddispersion was mixed with dipentaerithritol hexaacrylate (6 parts) as anelectron radiation curing resin, and the mixture was further stirred andfiltered to obtain the primer layer coating composition (i).

TABLE 2 Components of Primer Layer Coating Composition (i) Amount(parts) Acicular iron oxide powder (average 70 major axial length: 90nm; average axial ratio: 5) α-Alumina (average particle size: 80 nm) 10Carbon black (average particle size: G5 nm) 20 Vinylchloride-hydroxypropyl methacrylate copolymer 10 resin (—SO₃Na groupscontained: 1.0 × 10⁻⁴ eq./g) Polyester polyurethane resin 5 (—SO₃Nagroups contained: 1.0 × 10⁻⁴ eq./g) Methyl ethyl ketone 130 Toluene 80Cyclohexanone 65 Myristic acid 1 Butyl stearate 1.5

Preparation of Primer Layer Coating Composition (ii)

A primer layer coating composition (ii) was prepared in the same manneras in the preparation of the primer layer coating composition (i),except that no electron radiation curing resin was added.

Preparation of Backcoat Layer Coating Composition

The components of a backcoat layer shown in Table 3 below were dispersedwith a sand mill (residence time: 45 minutes), and the resulting liquiddispersion was mixed with polyisocyanate (8.5 parts), and the mixturewas stirred and filtered to obtain the backcoat layer coatingcomposition.

TABLE 3 Composition of Backcoat Layer Coating Composition Amount (parts)Carbon black (average particle size: 25 nm) 40 Carbon black (averageparticle size: 370 nm) 1 Barium sulfate 4 Nitrocellulose 28 Polyurethaneresin (—S0₃Na groups contained) 20 Methyl ethyl ketone 100 Toluene 100Cyclohexanone 100

Production of Magnetic Sheet

Each of the non-magnetic substrates shown in Table 4 below was used incombination with the coating compositions shown in Table 5 below.Firstly, the primer layer coating composition (i) or (ii) describedabove as applied to the non-magnetic substrate so that the resultingprimer layer could be 2 μm in thickness after being dried andcalendered. Simultaneously with this application, the magnetic coatingcomposition (I) was applied to and superposed on the primer layer sothat the resulting magnetic layer could be 100 nm in thickness afterbeing dried and calendered. The primer layer and the magnetic layer weredried while being oriented in the lengthwise direction. Thus, the primerlayer and the magnetic layer were formed. In this regard, when theprimer layer coating composition (i) containing an electron radiationcuring resin as the binder was used, the primer layer and the magneticlayer were exposed to an electron ray of 4 MRad after being dried.

Next, the above-described backcoat layer coating composition was appliedto the other surface of the non-magnetic substrate having the magneticlayer formed on one surface, so that the resulting backcoat layer couldbe 700 nm in thickness after being dried and calendered, and then, thebackcoat layer was dried.

The magnetic sheet, which had the primer layer and the magnetic layer onone surface of the non-magnetic substrate and had the backcoat layerformed on the other surface thereof, was specular finished with afive-staged calender (70° C. under a linear pressure of 150 Kg/cm); andthe resulting magnetic sheet was wound onto a sheet core, and was agedin this state under an atmosphere of 60° C. and 40% RH for 48 hours.

TABLE 4 Thickness YMu YD2* Type Material (μm) (GPa) (GPa) YD/YM AAromatic polyamide 3.1 15.2 16.0 1.05 B Aromatic polyamide 3.1 13.8 15.81.14 C Aromatic polyamide 3.1 15.9 13.9 0.87 D Aromatic polyamide 3.115.9 13.0 0.82 E Aromatic polyamide 3.1 13.1 15.9 1.21 F Aromaticpolyamide 2.2 15.2 16.0 1.05 Notes: 1) YD: Young's modulus in widthwisedirection 2) YM: Young's modulus in lengthwise direction

The Young's moduli of the magnetic sheet thus obtained in the lengthwiseand widthwise directions were calculated under the following conditions.

Young's Modulus

A tensile tester equipped with a precision displacement meter was usedto pull a test sample in the shape of a ½ inch square, and a Young'smodulus was calculated from the elongation of the test sample found whenthe distortion thereof was 0.3%.

Next, the magnetic sheet was cut into a tape with a width of ½ inchunder the cutting conditions shown in Tables 5 and 6, using the cuttingdevice shown in FIG. 3. Thus, the magnetic tape was obtained. In thisregard, an edge position control (or EPC) was disposed in the proximityof the cutting means so as to adjust the fluctuation of the magneticsheet in the widthwise direction, so that the web tension could bechanged with the use of the feeding roll, the take-up roll and theadjusting rollers, to adjust the fluctuation of the magnetic sheet inthe widthwise direction.

The resultant magnetic tape was evaluated in the center line averageheight (Ra) and friction coefficient of the cut edge thereof, and wasalso evaluated in linear miming performance and flaking therefrom. Theresults are shown in Tables 5 and 6.

Center Line Average Height (Ra) of Cut Edge

The cut edges of the magnetic tape were fixed with a jig, and theroughness curve of the cut edge thereof was measured with a lasermicroscope [i.e., a real time scan type laser microscope 1LM21D with aHe—Ne laser, CW type (wavelength: 632.8 nm, and maximum output: 0.1 mW),manufactured by Lasertec], under the following measuring conditions:

Magnification of the microscope: 1,000

Resolution: 0.3 μm

Number of times of slow scanning: 8

Scanning time: 40 seconds.

A roughness curve was obtained from 10 points on the cut edge of sample,and the data of the resultant roughness curve were processed to find acenter line average height (Pa).

Friction Coefficient

An evaluation apparatus comprising guide rollers (made of SUS) for LTOand tension gauges at its both ends was used. The magnetic tape waswound onto the upper flange of the guide roller at an angle of 0.5degree thereto, and was fed at a rate of 1.2 m/ruin. A frictioncoefficient of the magnetic tape was calculated from a differencebetween the tensions found from the tension gauges at both ends of theapparatus.

Linear Running Performance

The magnetic tape was set in a LTO cartridge, and this LTO cartridge wasloaded on a LTO drive. Then, the magnetic tape was fed in a shuttle mode(simple feeding at a tape speed of 6 m/sec. without recording orreproducing). The running performance of the magnetic tape fed infrictional contact with the guide roller in the drive was visuallyobserved. The linear running performance of the magnetic tape wasevaluated based on the following criteria:

A: no vibration of the tape edge

B: vibration of the tape edge

C: bending of the tape edge

Flaking

The magnetic tape was set in a LTO cartridge, and this LTO cartridge wasloaded on a LTO drive, and 2,000 passes of feeding of the magnetic tapewas performed in a shuttle mode (tape speed: 6 m/sec.). After completionof the feeding, dirt on the flange portion of the guide roller wasvisually observed, and the flaking from the magnetic tape was evaluatedbased on the following criteria:

A: no remarkable flaking

B: blackish discoloration of the flange portion

C: black powdery dirt on the flange portion

TABLE 5 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Non-magnetic substrate Type A B CF A Magnetic coating composition Type I I I I I Primer layer coatingcomposition Type I i i i ii Magnetic Ew (GPa) 15.5 15.2 13.4 15.7 15.3sheet Et (GPa) 14.7 13.3 15.4 14.9 14.5 Ew/Et 1.05 1.14 0.87 1.05 1.05Entire thickness (μm) 4.4 4.4 4.4 3.4 4.4 Cutting Intermeshing depth (L)(μm) 0.08 0.08 0.07 0.08 0.08 conditions Surface roughness Thin blade(μm) 0.04 0.04 0.02 0.04 0.04 (P-V) Thick blade (μm) 0.03 0.04 0.02 0.030.03 Movement in width-wise (μm) 80 72 89 88 85 direction EvaluationCenter line average Height (Ra) (μm) 0.11 0.09 0.23 0.15 0.12 Frictioncoefficient 0.29 0.31 0.17 0.25 0.18 Linear running performance A B A BA Flaking A A B B B

TABLE 6 C. C. C. C. C. C. Ex. I Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6Non-magnetic substrate Type D E A A A A Magnetic coating compositionType I I I I T I Primer layer coating composition Type i i i i i iMagnetic Ew (GPa) 12.5 15.4 15.5 15.5 15.5 15.5 sheet Et (GPa) 15.4 12.614.7 14.7 14.7 14.7 Ew/Et 0.81 11.2 1.05 1.05 1.05 1.05 Entire thickness(μm) 4.4 4.4 4.4 4.4 4.4 4.4 Cutting Intermeshing depth (L) (μm) 0.070.08 0.04 0.11 0.08 0.08 conditions Surface roughness Thin blade (μm)0.02 0.04 0.04 0.04 0.06 0.04 (P-V) Thick blade (μm) 0.02 0.04 0.03 0.030.06 0.03 Movement in width-wise (μm) 70 105 81 81 84 105 directionEvaluation Center line average height (Ra) (μm) 0.06 0.27 0.07 0.27 0.270.31 Friction coefficient 0.34 0.13 0.33 0.13 0.13 0.09 Linear runningperformance C A C A A A Flaking A C A C C C

As can be seen from the results in Tables 5 and 6, a magnetic tape witha cut edge having a center line average height (Ra) of from 0.08 to 0.25μm can be produced by cutting a magnetic sheet in which the ratio of theYoung's modulus in the widthwise direction (Ew) to the Young's modulusin the lengthwise direction (Et), i.e., Ew/Et, is from 0.85 to 1.15,using the following cutting means, while fluctuation of the magneticsheet in the widthwise direction being kept to 100 μm or less. Thecutting means comprises thin blades and thick blades, wherein theintermeshing depth (L) of the thin blades and the thick blades is from0.05 to 0.1 mm, and the maximum surface roughness (P-V) of any of theside faces of the thin blades and the thick blades at which both theblades are intermeshed with each other is 0.05 μm or less. It is alsoknown that a magnetic tape which has a center line average height (Ra)within this specified range shows a smaller friction coefficient at itscut edge to the flange portion of a guide roller. Therefore, such amagnetic tape is superior in linear running performance. It is alsoknown that flaking from the magnetic tape having the above-specifiedcenter line average height (Ra), fed at a high speed on the flangeportions of guide rollers, is reduced. It is further known that amagnetic tape which includes a primer layer containing an electronradiation curing resin is further decreased in friction coefficient andflaking therefrom, as compared with a magnetic tape which includes aprimer layer containing a therplastic resin or a thermosetting resinalone. It is considered that the use of the electron radiation curingresin as a binder is effective to improve the cutting property of themagnetic sheet.

In contrast, a magnetic tape having a center line average height (Ra) ofless than 0.08 μm is reduced in flaking therefrom. However, thismagnetic tape shows a larger friction coefficient at its cut edge to theflange portion of a guide roller and thus lowers in linear runningperformance. It is considered that this is because the cut edge of themagnetic tape is excessively smoothened to increase the contact areabetween the cut edge of the magnetic tape and the flange portion of theguide roller. On the other hand, a magnetic tape having a center lineaverage height (Ra) of more than 0.25 μm is decreased in frictioncoefficient, but is increased in flaking therefrom. It is consideredthat this is because too rough a cut edge of the magnetic tape wouldlocally strike at its raised portions against the flange portion of aguide roller.

What is claimed is:
 1. A magnetic tape having an entire thickness offrom 2 μm to 8 μm produced by cutting a magnetic sheet with a widebreadth into a tape with a predetermined width using a cutting device,wherein the magnetic sheet comprises a magnetic layer with a thicknessof 10 to 300 nm containing magnetic powder and a binder formed on onesurface of a non-magnetic substrate, and a primer layer comprisinginorganic powder and a binder having a thickness of 0.1 μm to 3.0 μmbetween said magnetic layer and said non-magnetic substrate, and whereina center line average height (Ra) along a roughness curve of a cut edgeof the magnetic tape in a thickness direction is from 0.09 to 0.23 μm.2. The magnetic tape according to claim 1, wherein the center lineaverage height (Ra) along a roughness curve of a cut edge of themagnetic tape in a thickness direction is from 0.09 to 0.12 μm.
 3. Themagnetic tape according to claim 1, wherein said binder of said primerlayer contains an electron radiation curing resin.
 4. The magnetic tapeaccording to claim 1, wherein a friction coefficient between a cut edgeof the magnetic tape and a flange portion of a guide roller is from 0.18to 0.31.